Can I lower temperature by underclocking?

Sokonomi

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Like it says on the tin, can I achieve lower temperatures by underclocking my CPU?

I have an Intel i7 920 processor (base 2.66Ghz) that I would like to start using in a small case in the back of my meter closet to handle my backups and downloads. The space is a bit cramped and there is little to no ventilation, so keeping temps low is of the highest importance.

Would it be a good start if I underclocked it and dropped the vCore as low as it can run stable? If so, what would be a good starting point as far as clockspeed and voltage are concerned?

I'm using an AIO single 120mm radiator watercooling unit (push/pull). Would that be preferable in such conditions or is aircooling going to be better?
 
If you're going to be sticking it into a closet with the door closed, the type of cooling you use on the computer is pretty irrelevant. Think of it this way - the computer is taking energy from the electrical outlet, turning it into heat, and pumping it into the closet air. The heated air in the closet then slowly dissipates out through cracks along the edges of the door. The hotter the air gets, the more quickly it dissipates. So the computer will heat up the air until the rate that heat is flowing out the door matches the rate that the computer is converting electricity into heat. Only these two rates matter. The method you use to remove heat from the computer doesn't matter.

If you water-cooled and somehow ran the tubes out of the closet, so the radiator was in air that has good circulation and temperature control (either mixes with outdoor air, or is air conditioned), then that would work.

Underclocking / undervolting would help. But in my experience the first-gen Core i CPUs tended to run very hot even when underclocked / undervolted. Sandy Bridge is when Intel got serious about reducing idle power consumption. An i7-970 system uses over 100 Watts when idle.

http://www.tomshardware.com/reviews/overclock-core-i7,2268-10.html

If you live in the U.S. and pay the national average of 12 cents/kWh, then every Watt consumed by a device left on 24/7 translates into almost exactly $1 per year. So a system that idles at 109 Watts will cost you $109 in electricity every year. A small NAS like a Synology or QNAP can easily handle backups and downloads, and will only burn about 8 Watts. Yes it'll cost you about $400, but based on the huge difference in power consumption, it'll take you only 4 years of use to earn back that money via saved electricity (even less time if electricity costs more than 12 cents/kWh where you live). Beyond that, the NAS will be cheaper than re-using your old i7 system. And the lower power consumption (which is the same as the rate electricity is converted into heat) means you may be able to just stick it in the closet and close the door without having to worry about the closet air overheating.
 

Sokonomi

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I wasn't planning on keeping it running 24/7, but I guess ill just have to pop the door on sundays when it comes on. It powers up at 12 o clock mid day to grab backups from all machines in the house, then downloads all the new TV shows from that week and deposits them onto an external drive hooked to my router, then shuts down again a few hours later.

A Qsnap/Syn NAS isnt an option, as I understand those things run on linux, and if there's one thing I absolutely despise in this world...

Thanks for the explanation though, I didn't realize these CPUs were so awful at power consumption. (I figured they would step down to almost nothing when Idle).